1. Field of the Invention
The present invention relates to a silver master plate recovery solution for recovering the ink receptivity of an image area on silver salt photographic plates for offset printing by reducing the silver oxide layer of the image area.
The silver salt photographic plate for offset printing (hereinunder referred to as "silver master plate") is a plate material produced by coating the surface of a support such as paper and plastic film with silver halide. And by electro diffusion transfer process this plate forms a smooth silver surface which constitutes a lipophilic image area for offset printing.
2. Technology Review
The surface of the silver master plate is divided into image areas and non-image areas as in a general metal photographic plate. The image area, which is a smooth metal silver surface (the polarity is theoretically zero), is lipophilic without any affinity for water. In other words, the printing area is receptive to ink. The non-image area is made of gelatin having a polarity and is hydrophilic without any affinity for oil. In other words, the non-image area is repellent to ink. In order to perform offset printing, it is necessary to maintain the polarity of the image areas in a low state and the polarity of the non-image areas in a high state.
The silver surface, which constitutes an image area on a silver master plate, begins to be oxidized immediately after plate making. Since silver oxide has a high polarity, the polarity of the oxidized image area becomes too high to maintain the lipophilic nature of the image area. As a result, the sensitivity to ink is lowered, there is a possibility of causing troubles such as non-uniformity of printing on a printed matter.
Accordingly, the guaranteed stock life of a conventional silver master plate is substantially 8 hours, which brings about the following problems:
(1) the operation of offset printing is troublesomely restricted by time, and (2) a silver master plate which has passed the stock life becomes unusable and is discarded.
It is considered that since such troubles are caused by the silver oxide layer in the image area, they will be eliminated by reducing the silver oxide layer by a reducing agent (silver master plate recovery solution).
However, no silver master plate recovery solution which recovers the lipophilic nature of the image area by reducing the silver oxide layer has been known.
This is because no conventional reducing agent can satisfy the following conditions (a), (b) and (c):
(a) To have a reducing ability in the pH range of 3 to 6.
In performing offset printing, it is inevitable that ink and water (fountain solution) are kneaded at a high speed by various rollers on the silver master plate. At this time, if the pH value of the fountain solution on the silver master plate is too high, the ink on the plate would be saponificated into a state of being compatible with water, namely, being emulsified. On the other hand, if the pH value of the fountain solution on the silver master plate is too low, there is a possibility of causing a phenomenon of roller stripping (ink fails to adhere to and distribute uniformly on the metal rollers) or chalking (ink pigment is not properly bound to the paper and can be easily ruffed off). It is therefore generally necessary to adjust the pH value of the dampening water on the silver master plate within the range of 3 to 6.
(b) To have a equivalent reducing ability in the pH range of 3 to 6.
It is difficult to fix the pH value of the fountain solution in a practical offset printing operation. Therefore, if the reducing ability of a reducing agent is controlled by the variation of the pH value of the print-immersing water, the reducing agent is not suitable for the practical offset printing.
(c) Not to make the image area (silver surface) porous after reduction.
According to the investigation undertaken by the present inventors, an aqueous hydrazine solution has been selected as a silver master plate recovery solution that satisfies the conditions (a) and (b). However, when the silver oxide layer on an image area is reduced by hydrazine, the surface of the image area becomes porous. The surface then physically produces a polarity and, as a result, water is physically adsorbed on the surface of the image area. In other words, the lipophilic nature (receptivity to ink) of the image area is lowered.